23.5.4 How to Optimize SSL Performance Using the SMTP Proxy

Most sites should not use the SMTP proxy as it adds additional latency
to the SMTP protocol. However, a large-scale site which makes heavy use of
SSL to protect SMTP connections may wish to maximize their investment in SSL
accelerator hardware by performing all SSL operations for all protocols on
a server which does nothing other than SSL and proxy. The SMTP proxy allows
SSL to be processed by a front end proxy server while the mail queues are
on a separate MTA machine. This way hardware optimized for each task can be
separately configured and purchased.